1. Field of the Invention
The present invention relates to a circuit for the detection of fuses with vertical oxide fusing.
Fuses are used in integrated circuits, for example for the definition of functions or access paths, or to fulfil an access-locking function in tamper-proof circuits. They have two possible states: the initial state, when they are intact, and the irreversible, final state when they are fused or blown.
2. Description of the Prior Art
The fuses currently used in integrated circuits may be made:
- of metal. Their drawback then is that they can be short-circuited.
- of polysilicon. Their drawback then is that the blown fuse can grow back.
- on the basis of two EEPROM cells. The blank or virgin cells are in a same unknown state. One may be only erased and the other may be only programmed. The blowing is obtained, for example, by activating the programming of the double cell. Since only one cell may be programmed, the combined OR output of the two cells changes state. But a circuit such as this takes up a great deal of space.
- with one EEPROM cell, the blowing of which consists in the evaporation of its oxide tunnel by the sending, for example, of one or more programming signals with very steep front edges. However, the writing and reading cells are then complex, and a fuse such as this is bulky
There is a known association of a very thin oxide zone with a transistor. The purpose of the oxide zone is to change the state of the transistor. This type of a use is described in the U.S. Pat. No. 4,701,780.
This use is substantially equivalent to that of an EEPROM cell with tunnel oxide fusing.